Shim

1. A small piece of data inserted in order to
achieve a desired memory alignment or other addressing property.
For example, the PDP-11 Unix linker, in split I&D (instructions and
data) mode, inserts a two-byte shim at location 0 in data space so
that no data object will have an address of 0 (and be confused with
the C null pointer). See also loose bytes. 2. A type of
small transparent image inserted into HTML documents by certain
WYSIWYG HTML editors, used to set the spacing of elements meant to
have a fixed positioning within a TABLE or DIVision. Hackers who
work on the HTML code of such pages afterwards invariably curse
these for their crocky dependence on the particular spacing of
original image file, the editor that generated them, and the
version of the browser used to view them. Worse, they are a poorly
designed kludge which the advent of Cascading Style Sheets
makes wholly unnecessary; Any fool can plainly see that use of
borders, layers and positioned elements is the Right Thing (or
would be if adequate support for CSS were more common).

A shim is a thin piece of wood or metal. I'ma talk about wooden shims first because those are the only kind I've used. These are very useful when you want to disguise the fact that you built a crappy truss. See, when a gap is left in your truss, either between the kingpost and the bottom chord, an upright and the bottom chord, an upright and intersecting webs, or a web and a top chord, it is necessary to close that gap (to protect the reputation of yourself and your truss company). To do this, you take a thin piece of 2x4, usually no more than a half an inch wide, and jam it into your gap. Then, you plate the joint as usual.

If you need a shim that's wider than a half an inch, or you find yourself using shims often, you have bigger problems on your hands. I would suggest reevaluating the whole truss and setting it up all over again. You may have had warped chords when you set it up originally.

Shims are usually just cut to fit when the need for one arises. However, you can buy boxes of pre-cut shims at just about any hardwarestore. Those shims are long pieces of wood, like 8 to 10 inches long, with an angled cut to them so it gets gradually wider, that way you can chop the shim to the width you need.

When my family was building our house, we used some shims under our trusses after we set them because apparently the outside walls of our house were taller than the inside walls and we were forced to compromise with shims.

Other kinds of shims, i.e. metal ones, are used in fields other than carpentry. For example, let's say the handlebars on your bicycle are loose and you don't have a screwdriver with which to tighten them. An option would be to jam a thin strip of flexible metal (like a piece of an aluminum can) between the support thingy and the handlebar tightener thingy, thus reducing the space between the two and making the bars tighter.

A shim is anything made out of any material designed to tighten a loose fit. Washers can be used as shims, steel plates, wood, or anything. Wood shims are used to hang doors and set cabinets. Usually those shims come in the shape of slim wedges, so they can be moved for a more precise fit. In fitting a door, they can be used in multiples for a tighter fit on wider surfaces. I use shims all the time to secure boxes cut into finished masonry walls, pounding flat small pieces of electrical conduit in order to fill the wall.

In the classic book Zen and the Art of Motorcyle MaintenanceRobert M. Pirsig cuts a shim from a pop can for his friend's BMWmotorcycle. His friend is appalled that such a humble device could be used to perfect such a good example of German engineering. Germans are famous for their precision, Americans for making it work. When the robotically constructed Honda cars arrived in America mechanics were amazed that they needed no shims to fit the doors, unlike the American cars they were so used to. Yet even precise tolerances may relax with use and wear. To Pirsig the engineer who designed the BMW would have appreciated his use of the pop can. His homemade shim is perfect, an essentially free part cut from a soft metal that will serve its intended purpose beautifully.

Softness is a desirable property in a shimming materials. You want a bit of give, and a bit of springiness so it will hold tight. But a shim is made out of anything you have handy. Shimming is an important part of any handyman's bag of tricks.

thanks to dannye for pointing out that Honda's arrival in American revolutionized the tolerances used in american, and European, cars. Today tight fits are the norm, in the old day we pieced things together.

Note: This example does not provide an accurate rod speed. This was intentionally done because actual control rod speeds are classified.

Note: currently working on a node for control rods that will explain why shimming is necessary.

So why is shimming necessary?

As the control rods are withdrawn from the core, fewer and fewer neutrons are absorbed by the rods and more neutrons are absorbed by the 235U fuel. At the point where the number of neutrons produced by the fuel is equal to the number of neutrons absorbed by the fuel is known as criticality. This is a good thing. A reactor must be critical to produce power. So, whenever you see a movie that exclaims, "The reactor's gone critical!" you know that they don't know what they're talking about. The term they were looking for was prompt critical.

Once the control rods have been pulled to criticality, then they must be pulled, or shimmed further. The reactor is still not yet at a point where it can produce electrical power. Reactor power is still at a level insufficient for producing steam. More shimming is required to raise the temperature of the reactor.

Once temperature has been raised to the normal operating range, occasional shimming is still required. Temperature will slowly drift because of the buildup or burning off of neutron absorbing poisons such as 135Xe.